Electronic Components Datasheet Search |
|
LM4702B Datasheet(PDF) 11 Page - National Semiconductor (TI) |
|
|
LM4702B Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 15 page Application Information (Continued) bias point on the amplifier’s input terminal. The resulting -3dB frequency response due to the combination of C IN and R IN can be found from Equation (5) shown below: f IN =1/(2 πR INCIN) (Hz) (5) With large values of R IN oscillations may be observed on the outputs when the inputs are left floating. Decreasing the value of R IN or not letting the inputs float will remove the oscillations. If the value of R IN is decreased then the value of C IN will need to increase in order to maintain the same -3dB frequency response. AVOIDING THERMAL RUNAWAY WHEN USING BIPOLAR OUTPUT STAGES When using a bipolar output stage with the LM4702 (as in Figure 1), the designer must beware of thermal runaway. Thermal runaway is a result of the temperature dependence of Vbe (an inherent property of the transistor). As tempera- ture increases, Vbe decreases. In practice, current flowing through a bipolar transistor heats up the transistor, which lowers the Vbe. This in turn increases the current again, and the cycle repeats. If the system is not designed properly, this positive feedback mechanism can destroy the bipolar tran- sistors used in the output stage. One of the recommended methods of preventing thermal runaway is to use a heat sink on the bipolar output transis- tors. This will keep the temperature of the transistors lower. A second recommended method is to use emitter degenera- tion resistors (see Re1, Re2, Re3, Re4 in Figure 1). As current increases, the voltage across the emitter degenera- tion resistor also increases, which decreases the voltage across the base and emitter. This mechanism helps to limit the current and counteracts thermal runaway. A third recommended method is to use a “Vbe multiplier” to bias the bipolar output stage (see Figure 1). The Vbe multi- plier consists of a bipolar transistor (Qmult, see Figure 1) and two resistors, one from the base to the collector (Rb2, Rb4, see Figure 1) and one from the base to the emitter (Rb1, Rb3, see Figure 1). The voltage from the collector to the emitter (also the bias voltage of the output stage) is Vbias = Vbe(1+Rb2/Rb1), which is why this circuit is called the Vbe multiplier. When Vbe multiplier transistor (Qmult, see Figure 1) is mounted to the same heat sink as the bipolar output transistors, its temperature will track that of the output transistors. Its Vbe is dependent upon temperature as well, and so it will draw more current as the output transistors heat it up. This will limit the base current into the output transis- tors, which counteracts thermal runaway. www.national.com 11 |
Similar Part No. - LM4702B |
|
Similar Description - LM4702B |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.NET |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |